A VCO device is used in, for example, a broadcast receiver and a communication receiver and used as a circuit that generates a local frequency for tuning any high frequency signal.
FIG. 20 is a circuit block diagram showing a conventional VCO device. In FIG. 20, VCO circuit group 104 oscillates signals of frequencies corresponding to control voltage Vt applied to a frequency control voltage terminal. Current source circuit 105 sets a driving current of an oscillation transistor (not shown) that is an element of VCO circuit group 104. Signal selecting means 103 selects an output signal from VCO circuit group 104 and outputs local signal fvco to a local signal output terminal. PLL 106 frequency-divides local signal fvco selected by signal selecting means 103, compares a phase thereof with a phase of a reference signal, and outputs a voltage signal converted from a phase difference. Loop filter 107 smoothes the output signal taken out of PLL 106 and outputs control voltage Vt that controls an oscillation frequency of local signal fvco.
In a mobile receiver including a mobile telephone, it is useful to prepare a plurality of VCO circuits 104a, 104b and 104c, which oscillate in different frequency ranges, as shown in VCO circuit group 104. It is useful because such a circuit configuration is preferable in achieving downsizing of IC mounted on a mobile receiver or reduction of power consumption, in securing a normal operation at low power supply voltage and in obtaining excellent phase noise characteristics in a wide frequency range.
FIG. 21 shows a relation between frequency control voltage Vt supplied to VCO circuits 104a, 104b and 104c and the oscillation frequencies thereof. In particular, for frequency-converting a-high frequency signal of a wide frequency range for television broadcast receiver, etc. into a first IF signal, a VCO circuit is likewise required to oscillate a local signal of a wide frequency range. In order to satisfy the oscillation frequency of the local signal over a wide range, VCO circuit 104a plays a role in oscillating in a low frequency band as an oscillation frequency range. Furthermore, VCO circuit 104b plays a role in oscillating in a middle frequency band as an oscillation frequency range, and VCO circuit 104c plays a role in oscillating in a high frequency as an oscillation frequency range, respectively. Thus, since each of the VCO circuits plays a role respectively, a predetermined frequency range is oscillated, and thereby required electric characteristics can be obtained.
Note here that prior arts related to this kind is described in, for example, Japanese Patent Unexamined Publication No. 9-102752.
However, when the range of oscillation frequency is intended to be widened by using a conventional VCO device, as shown in FIG. 22, as is apparent from the relation of characteristics between the offset frequency and the phase noise of VCO circuits 104a, 104b and 104C, the signal oscillated by VCO circuit 104a that plays a role in oscillating in a low frequency band of the local signal has a relatively smaller phase noise as compared with VCO circuits 104b and 104c. 
This is because Q (Quality factor) of a resonance circuit built in the VCO circuit depends upon the frequency characteristics. As the oscillation frequency is higher, impedance of signal wiring of the circuit or stray capacitance provided to the signal wiring becomes unignorable, and thereby Q is deteriorated. When Q of the resonance circuit is deteriorated, the phase noise is generally increased.
As shown in FIG. 20, according to a conventional VCO device, current source circuit 105 for setting current of oscillation transistors (not shown) prepared for VCO circuits 104a, 104b and 104c was shared by the VCO circuits. Alternatively, when current source circuits 105 are connected individually, the currents thereof were set to substantially the same value.
In such a configuration, the current of current source circuit 105 is set so that required characteristics of, for example, VCO circuit 104c, which has the highest frequency range and is disadvantageous in lowering the phase noise, can be achieved. Therefore, VCO circuits 104a and 104b which are relatively advantageous in lowering the phase noise necessarily operate below the required characteristics of the phase noise characteristics. VCO circuits 104a and 104b satisfy the required phase noise. However, they have to supply undesired excess current, which may lead to an increase in power consumption.
When a VCO device is mounted on a mobile terminal including a mobile telephone, continuous usable time is limited by a battery as a driving power supply. Therefore, it is a very important problem to achieve low power consumption of a broadcast receiver and a communication transmitter/receiver mounted on a mobile terminal.